Scientists discover a surprising link between acorns and lyme disease

Researchers at the Cary Institute of Ecosystem Studies in Millbrook, New York, have tracked the ecology of Lyme disease for more than 35 years. Their new study, published in Proceedings of the National Academy of Sciences, synthesizes nine key findings from this long-term data set. The most striking pattern links oak tree acorn masting events to future tick abundance. When oak trees produce exceptionally large acorn crops, white-footed mouse populations expand the following year. Those larger mouse populations then support roughly 40% more nymphal blacklegged ticks the year after that, creating a two-year lag between a heavy acorn crop and a surge in the tick stage most responsible for transmitting Lyme disease to humans.

This chain of events upends the long-held assumption that white-tailed deer are the primary driver of tick numbers. Although deer provide blood meals for adult female ticks, the Cary Institute's full data set shows no statistically significant connection between deer abundance and subsequent nymphal tick abundance on their 2,000-acre research property in Dutchess County. "We had detected a weak positive relationship between deer and subsequent nymph abundance over short subsets of the data," said lead author Richard Ostfeld, "but the whole data set in its entirety shows no such thing." Instead, mouse abundance emerged as a much stronger predictor of nymphal tick numbers.

However, more mice do not reliably predict a higher percentage of ticks carrying the Lyme bacterium. Early data suggested infected nymph proportions would rise with mouse numbers, but that relationship vanished as more years accumulated. "It kept me up at night," Ostfeld said, "because it meant that my understanding of how the system works was incomplete or wrong." The explanation involves other forest hosts such as skunks, squirrels, and opossums. These animals feed many larval ticks but are poor reservoirs for Borrelia burgdorferi. When they are abundant, more larval ticks feed on non-infecting hosts, diluting the infection prevalence among nymphs even when mice are plentiful.

The study also complicates expectations about weather. Laboratory experiments suggested extreme heat or cold could kill ticks, yet field observations show ticks survive by retreating into soil and leaf litter. Paradoxically, warmer years overall tend to be followed by fewer nymphal ticks, even though warmer conditions generally favor mouse populations. These opposing effects make it difficult to predict how changing temperatures will ultimately influence Lyme disease risk. The findings collectively demonstrate that Lyme risk depends on a network of ecological interactions that is complicated but not entirely unpredictable, offering public health officials potential early-warning indicators such as acorn crops and mouse abundance.